MICRO- AND NANO- SIZED INCLUSIONS IN IAB DIAMONDS AND THEIR GEOLOGICAL IMPLICATIONS

The origin and formation of pure type IaB diamonds (with four atoms substituting for carbon atoms) are mainly mysterious. Here we systematically studied the micro- and nano- sized inclusions of type IaB diamonds by Raman spectroscopy, SEM and TEM. Around 50 diamonds show micro inclusions detectable by Raman spectroscopy, and a majority of them contain typical mineral inclusions from sublithosphere, such as walstromite, larnite, titanite, coesite, Ca-perovskite, jeffbenite (TAPP), ferropericlase, which might be fragments of transition zone or lower mantle. Syngeneic rhombus shape calcite inclusions have been found coexisting with walstromite. Moreover, methane has been detected in the fluid jacket in several inclusions coexisting with carbonate phases, implying a carbonate-rich fluid has been involved with deep origin during diamond forming process. Transmission electron microscopy (TEM) revealed nano-sized octahedron shaped negative inclusions in milky IaB diamonds. Some of them contain appreciable nitrogen and iron, together with crystallized sub-micro iron-rich inclusions. Elongated sub-micro negative inclusion has been observed with nano-sized iron-rich particle inside, in a diamond with micro-sized walstromite. Besides, high-density dislocation networks have been observed by TEM, suggesting the diamond has undergone severe deformation process. Evidence of the sublithospheric origin of a majority of IaB diamonds, and their interaction with deep hydrocarbon, carbonate fluids as well as Fe-rich fluids would be important to interpret chemical recycling by Earth dynamics.